feat(settings): Extensions tab — "watch this space" notice (T-456)

Built-in extensions are always on and there's no third-party install path yet,
so the Extensions tab is a notice rather than a toggle list — a toggle list
would let users brick the app by disabling load-bearing builtins. It explains
that installing/enabling/disabling arrives with third-party (Lua) extensions
and points at the records that pick it up (D-16 / T-8).

Fleshes out the builtin.extensions-ui stub via the custom-control hatch. Adds
two renderer guards — empty section/field labels render no chrome — so a
notice-only category shows just its card.

Tests: contributes the Extensions category + notice control; the notice renders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/extensions_ui/src/extension.dart

@@ -1,17 +1,39 @@
+import 'package:clide/builtin/extensions_ui/src/extensions_notice.dart';
 import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
 
-/// Tier-0 stub. Real implementation lands in a later tier; the extension
-/// is registered so the extensions-ui surface can list it as "installed,
-/// not yet implemented" and its id is reserved.
+/// Extensions settings tab (T-456). Built-in extensions are always on and there
+/// is no third-party install path yet, so the tab is a "watch this space"
+/// notice pointing at the records that track extension management (D-16 / T-8)
+/// rather than a toggle list. The real enable/install UI lands with third-party
+/// (Lua) extensions.
 class ExtensionsUiExtension extends ClideExtension {
   @override
   String get id => 'builtin.extensions-ui';
   @override
   String get title => 'Extensions UI';
   @override
-  String get version => '0.0.0-stub';
+  String get version => '0.1.0';
   @override
   List<String> get dependsOn => const [];
+
   @override
-  List<ContributionPoint> get contributions => const [];
+  List<ContributionPoint> get contributions => [
+    SettingsControlContribution(id: 'extensions-ui.notice-control', customId: 'extensions.notice', builder: (_) => const ExtensionsNotice()),
+    const SettingsCategoryContribution(
+      id: 'extensions',
+      category: SettingsCategory(
+        id: 'extensions',
+        title: 'Extensions',
+        iconName: 'puzzle-piece',
+        priority: 80,
+        sections: [
+          SettingsSection(
+            label: '',
+            fields: [SettingsField(key: 'app.extensions._notice', kind: SettingsFieldKind.custom, label: '', customId: 'extensions.notice')],
+          ),
+        ],
+      ),
+    ),
+  ];
 }